I have been having issues with my SE84DIY ever since I got it last October. The problem is that it distorts when the output goes over about 1/2 watt (8 ohm speaker). With a sine wave input, this translates into anything over 2 v rms (5.7 v ptp) measured across the speaker is distorted.

The problem is occurring on both channels.

All tubes have been replaced, and both sets of tubes behave the same way.

When the distortion is first heard (with a sine wave input) the input voltage at the grid of the input tube is 1.5v ptp (0.53 v rms) or about 4 times lower than the 2v input required for full output (as stated in the assembly instructions).

The distortion is first seen as the clipping of the top of the sine wave at the plate of the input tube. It is then propagated through the output tube, transformer, and into the speaker.

A secondary effect (that happens later as the volume control is increased further) that may shed light onto the problem is that as the input level increases (up to 2v) the power supply increases. As much as 50 vdc higher at a 2v input than with zero input. The waveform at the input grid of the output tube is shifted way negative...normally its a nice sine wave centered about zero volts, but with high input level its a clipped sine wave going only 4v positive and 40 to 50 v negative. This grid voltage is seen on both channels. I believe the large negative voltage on the grids of the output tubes is turning the output tubes off to a point where they aren't conducting and this reduction in current is why the power supply voltage is higher. I have no clue as to why such a large negative voltage is at the grid of the output tubes.

I have a sine wave generator, lots of volt meters, and an osscilloscope, so I should be able to setup, test, and report on anything further one might need to determine what the problem is.

All testpoints are within the specs as stated in the assembly instructions. I have not measured them to see how they change as the load changes from an 8 ohm to a 4 ohm speaker, but I can do that later if its important.

I suspect the spec you are thinking about is the 10.25v on the cathode of the output tube...Yes that's there, and any signal level up to 10.25 v peak (7.25 v rms) on the grid of the output tube produces clear, undistorted, output. Trouble is that 7.25v rms on the grid of the output tube only produces 2 v rms across an 8 ohm speaker which is only 1/2 watt. Both channels are doing this.

This 2.3 vrms across a 8 ohm load is only 0.66 watts. I would expect 3.8 vrms to get 1.8 w across a 8 ohm load.

The reason I suspect the transformers is that I don't think you can get any larger signal at the input of the output tube as you will exceed the voltage at TPC and that will turn the tube off. The only thing left is the transformer or too low a gain in the output stage.

If you installed the volume control directly on the circuit board make sure the shell of the pot is grounded. Otherwise it becomes an antenna.

Point of interest, the output stage of the Zen amps including this kit do not clip. It is the input stage that clips.

Another point of interest, the input jacks are backwards so if you apply your 1KHZ signal to only one input and measure the output jack closest to it, you'll be measuring a greatly reduced signal because your on the wrong output jack. You would think that there would be no signal, however crosstalk between channels creates the condition.

I'm still trying to exactly replicate your results, I'll report back soon. I don't believe there is any issues with your output transformers. BTW, which output transformers do you have - EM406 ?

Got my new scope set up this weekend and had no problems simulating your results. I didn't have to do anything. I came up initially with .89 watts across an 8 ohm load if I got generous and .69 if I took the conservative approach. Yes, I was a bit surprised. Three basic variables seem to be at play here... A) The 6P15P-EV tubes bias up a bit differently than the 6P15P aka SV83. B) The solid state regulation yields a higher B+ voltage. C) The circuit board itself.

All of these combined effect the numbers quite a bit more than I expected. Just goes to prove that nearly identical circuits can perform quite differently. I assumed it was much closer to the SE84C because in listening tests it showed no real evidence of being noticeably different.

In any case I can recommend the following tweeks to improve the specs, and have listened to it. It's still a wonderful sounding amplifier, but you'll have to decide for yourself if the modifications make it sound better to you or not.

1) Remove the 2.7K resistor from the cathode of each side of the input tube, leaving only the 1.5K installed on each side. This will slightly lower the gain of the input stage, but at the same time allow it to take a hotter signal before clipping as well as maintain a cleaner sine through both stages.

2) Change the 150 ohm bias resistor for the output stage to a 500 ohm. This will raise the bias of the tubes from around 9 volts (that's what mine were) to just over 12 volts. Depending on the tubes you use the optimal value could fall between 200 ~ 500 ohms.

This will get you 3.25VRMS across an 8 ohm load with 0.40 amps yielding 1.32 watts

The input signal at the wiper of pot is 1.03VRMS. Of course you'll seldom ever see a signal higher than that from the back of CD player when it's playing music.

NOTE TO ALL: Regarding the above modifications... keep in mind that the RMS voltage of the music signal at the first stage is typically around 250 millivolts at a normal listening level. True the above modifications double the power of the amplifier. However, at normal source levels the dynamics and presence of the amplifier increase proportionately as you drop from 500 ohms on the output stage bias resistor back down to 150 ohms.

Put another way, the amp is likely to sound better in most situations when built in it's stock form.

Steve, could you split the output tube bias by using one resistor on each output tube? Or perhaps using a capacitor to bypass the bias resistor? This would reduce local negative feedback and low frequency cancellation caused by the larger bias resistor value.

I love the great depth of image the shared bias resistor gives. Would a 500 ohm potentiometer in series with the bypass capacitor allow tuning for good dynamics / presence vs imaging?

The cathode resistors can be split if you double the value. Also, there already is a bypass cap there.

Using a pot in a self bias circuit in place of a fixed resistor generally doesn't work because it becomes unstable and or noisy when it's moved. You would have to use a stepped attenuator built with 10 watt resistors...

Alternately you could create a fixed bias with a negative voltage on the grid and use pots to adjust the negative voltage. The efficiency of the amp would increase and significant power gains would be seen.

I'm still having problems. I made the changes as you suggested and the power output did increase, but its still only about 1 watt (.9) which is about half of what the amp should do...It is rated at 1.8 watts into 8 ohms, right? Here are the before and afters (1hkz input turned up until clipping just begins at the 8 ohm load resistor), hopefully you can tell me where they go bad.

I see that the power supply voltage has gone up quite a bit because of the lower bias current due to the increased cathode resistors.

Quite frankly, I don't see how you can get 1.8 w out of this. It would mean 10.7 v ptp across the 8 ohm load which would require 750 v ptp into the transformer (750 ptp with a 400 v power supply????) which would be around 40 v ptp at the grid of the output tube which would require the output tube cathode to be at 20 vdc.

Dear Steve,This week I finished the Decware ZKIT1. It were functioning and sounding well, The Kit runs with the 6N15N-EB power tubes.One thing were noticed me. When you measure, the output signal with a tests signal 1 KHz, 5 ohm load, 1.0 volts RMS on the output, the distortion are rather high, -30 dB. You can see this also on the scope.The sinus wave on the output was clipping.When the output raised to 1.2 volts the distortion firstly strongly reduced to -36 dB and raised (normally) -35. -30, -25 etc, when you ongoing raise the output. When you pointed pin 6 from the output tube to ground, this problem is solved. The distortion after this change gives -36 dB, 1.0 Volt RMS. You can see on the scope the tube is not clipping anymore in the 1 volt RMS output area. Before I modified the Decware Zkit, I will first hear the Kit a longer time.I’m curious what your experiments’ are whit this suggestion.

Since you're still trying to get 1.8 watts out of it I guess I wasn't clear in my prior posts. I basically duplicated your results with .69 watts into 8 ohms, which I indicated came as a surprise to me too. I speculated the reason for the difference was likely the circuit board itself. The 1.8 watt rating was assumed to be correct since it is the same parts and circuit as an SE84C. As we are seeing it is apparently not measuring the same as it's point-to-point tube rectified counterpart! Interestingly enough, in side by side listening tests the difference is hard to find.

Just for kicks, I measured my Zen Head at the same time and came up with 2V on the output at 0.25 amps into 8 ohms which is 0.50 watts. So I hooked it to a test speaker with some music and it could barely drive it. In fact my cell phone speaker is louder. I then took the ZKIT1 and hooked it to the same test speaker and it was like 10 times louder!

I suspect the shield on the circuit board can be improved so I will make some mods on the next batch of boards and re-measure. If the results do not change I will alter the rating from 1.8 watts to whatever it ends up being.

Now we are getting somewhere! Thanks Henk. I connected pin 6 of the output tubes to pin 3 (not ground) and the output before clipping across 8 ohms changed from 7.5 vptp (.88 w) to 8.7 vptp (1.18 w). This is an increase of 34%, yea!

Its getting almost loud enough so I can listen to it...trouble is that it sounds so darn good that I naturally want to turn it up, and when it won't go up any more its so frustrating I just don't listen to it.

I think my next step will be to put in a negative bias to the grid of the output tube(s) so I can do away with, or at least reduce, the common cathode resistor, which is now at 500 ohms.

After that, I have plans to try a "boosted triode", inspired by an EDN Design Idea published in June of 2003. www.edn.com/article/CA302240.htmlif anyone is interested. I will let you know what happens.

Connecting pin 6 (grid 3) to pin 3 (cathode) on a 6P15P-EV makes more sense. An EL84 has grid 3 and the cathode internally connected leaving pin 6 unused, where a 6P15P has them separated. This is why you see a performance gain similar to using an EL84.

Dan - if you're getting close to the volume level your after, try running a preamp with gain in front of the amp, and turn the volume on the amp down a bit, then turn the volume on the preamp up a bit to compensate.

Dear Steve,After my remarks (02/21/09 ) about pin 6 from the tube 6N15N-EB, I connected pin 6 with a 120 K carbon resistor to ground.120 K is sufficient for the grid load discharged and prevents distorsion and compression, but gives a nice better sound.In your last modification information you mention the use of an non-polarized film cap. What type and value you use this?.

Thanks for your response, If you send me a couple capacitors, I am grateful. My address is in the Netherlands.I can still not through the forum mail at this time.If you want to email me a message, I can give you my address. Thanks,